1. Field of the Invention
The present invention relates to a code violation detecting circuit, and more specifically to a circuit for detecting a code violation in the transmission of a signal transmission system in accordance with the CCITT-I430, regardless of a connection polarity of the transmission line.
1. Description of Related Art
Recently, the demand for digital data communication is increasing year after year. Under this circumstance, the CCITT (International Telegraph and Telephone Consultative Committee) advances an international standard for ISDN (Integrated Services Digital Network). One typical standard is a so called "alternate mark inversion" (AMI) signal.
In this AMI signal, a first binary logic level of for example "0" is alternately expressed by a first polarity signal and a second polarity signal different from each other in polarity, and a second binary logic level "1" complementary to the first binary logic level "0" is expressed by a signal different in potential level from each of the first and second polarity signals. The code violation is realized by continuous occurrence of two same polarity signals within a predetermined period starting from a start of each frame without interleaving the other polarity signal.
In the prior art, a detection of the code violation has been performed by detecting the continuous occurrence of two same polarity signals within the predetermined period without interleaving the other polarity signal, since the S point is not necessarily connected in a proper condition. However, the continuous occurrence of two same polarity signals without an intervening signal of the other polarity will appear, not only at the code violation portion in each signal frame, but also at a boundary between each pair of continuous frames, for example given by a framing bit and a data bit which is included in the just preceding frame and which has the the same polarity as that of the framing bit. As a result, only the correct code violation cannot be detected in the prior art violation detection circuit.